The objectives of this proposal are to characterize and quantify mechanisms of genetic variation in human somatic cells, and to elucidate features of the genetic organization of the major human histocompatibility complex (HLA), and the structure and function of its gene products. These objectives will be accomplished by the induction, isolation, and characterization of clones variant for gene products of HLA loci in diploid human lymphoid cultures. Besides characterizing spontaneous mechanisms of genetic variation in somatic cells, we will investigate the types of genetic lesions induced by mutagens.